Yu-Kun Hu, from Lanzhou University, discusses his article: Leaf functional traits predict timing of nutrient resorption and carbon depletion in deciduous subarctic plants
Nutrient resorption from senescing leaves is an important way that plants maintain their nutrient balance, meeting more than 1/3 of their total nutrient requirements. Thus, these nutrients are critical for plant growth and survival in nutrient-poor ecosystems. Additionally, the timing of nutrient resorption also matters. That’s because late resorption allows plants to have more photosynthetic carbon gains, but makes them more susceptible to fall frost damage. In contrast, early resorption lets plants avoid frost damage and nutrient loss, but decreases carbon gain.
In seasonal environments such as the subarctic region, timing of nutrient resorption becomes especially important for plants because of strong temporal variability in the first fall frost. Thus, variation in resorption timing can have great impacts on resorption efficiency and plant fitness.
Then, how large is the variation in resorption timing of plants? What determines the timing of nutrient resorption? All these questions haven’t been studied yet.
Hypotheses
We hypothesized that there are two alternative strategies regarding the timing of element resorption (early-and-steady versus late-and-fast resorption) in seasonal environments. We also hypothesized that these resorption timing strategies were closely related to plant growth form and resource economics traits.
Our study
We chose four types of subarctic ecosystems (shrubland, polygon heath, mire, and riparian forest) around Abisko Scientific Research Station, Sweden. Across these ecosystems, 22 predominant plant species from two growth forms were selected: 9 deciduous woody species and 13 herbaceous species.
We monitored the nutrient dynamics of these plant species during leaf senescence. To achieve this, we performed five sampling campaigns from July to September. Then we calculated timing of resorption as the Julian calendar day of 50% of element resorption (T50) and examined the variation in resorption timing and its linkage to plant growth form and leaf economic traits.
Our findings
We observed substantial variation in the timing of element resorption across subarctic plant species. Nitrogen resorption occurred from day 213 to 254, while phosphorus resorption was from day 211 to 261 and carbon resorption was from day 214 to 260. On average, the resorption of N and P, and depletion of C, were 13, 12, and 19 days, respectively, earlier in herbs than in woody plants. This suggests that herbaceous plants adopt a strategy of steady-and-slow resorption and woody species have late-and-fast resorption.
In addition, we found that the timing of element resorption aligned with key leaf traits from the plant economic spectrum, one of the most acknowledged strategy axes in plant science. That is, plant species with more conservative traits (larger leaf mass per area and leaf C content, and lower leaf N and P content) generally had later nutrient resorption.
Our study focuses on plant nutrient re-use from a new perspective: the timing of resorption. The great variation of resorption timing highlights the significance of timing for plant nutrient budgets and carbon economy, and potentially even plant fitness. The observed differences in timing between plant growth forms has important implications for the risk of frost damage and nutrient cycling linked to functional changes within plant communities in cold regions under future climate changes.
I would also like to thank my coauthors, Michelle Schollert, Rien Aerts, Richard S. P. van Logtestijn, James T. Weedon, and Johannes H. C. Cornelissen, who contributed to the exploration of this topic.
